The main advantage of CMOS technology over old technologies of CCD or amorphous silicon thin film type is the possibility of placing at the level of each pixel an electronic circuit associated with a radiation detection element; the detection element supplies an electrical charge current according to the stream of X photons received by the pixel; the electronic circuit processes this charge current before supplying at the output of the matrix processed information rather than a quantity of raw charges. For simplicity, the term “photodetector” will be used hereinafter to denote this photon stream detection element, regardless of whether a direct or indirect conversion structure is used, because the main interest is in the circuit for reading the electrical charges obtained from the conversion structure and not the conversion structure itself.
Thus, with a CMOS technology, instead of collecting and transferring to an output register a quantity of electrical charges generated in each pixel, these charges can first of all be converted into an analog current or voltage level and the current or voltage transported outside of the matrix by successively addressing each row of the matrix. Even better, the charges can be converted into digital signals within the pixel itself, to more easily transport the result of the detection towards the outside of the matrix, still by successively addressing the various rows of the matrix.
FIG. 1 represents a schematic diagram of a matrix detector for radiology, in the case (taken by way of example) of an indirect conversion structure: the matrix detector comprises a matrix 10 of photosensitive pixels, a scintillating layer 12 converting the X- or gamma-rays into light rays in the spectrum to which the photodetectors 14 of the matrix are sensitive, and a processing circuit 16 associated with each pixel. The processing circuit is essentially a circuit for reading and converting charges and, for simplicity, it will hereinafter be denoted charge reading circuit 16.
In the case of radiology, two main types of charge reading circuit can be incorporated at the level of each pixel:                an integration circuit which integrates in an integration capacitor the sum of the charges produced by the lighting of the pixel during a period of exposure to the X- or gamma-rays; the output of the pixel is an analog voltage proportional to the X or gamma energy received during this period; however, it is also possible to provide digital data representing this energy, as will be seen below;        or a photon counting circuit which entails converting the burst of electrical charges resulting from each photon into a voltage pulse; for this, the amplitude of this pulse is compared to a reference voltage which represents an energy threshold below which it is considered that there has been no photon impact; the energy pulses above this threshold provoke the delivery of a pulse which increments a digital counter at the output of the pixel; this counter contains, at the end of the exposure period, the number of photons received during this period and having an energy greater than the threshold.        
The advantage of the photon counting method is that it directly provides digital information at the output of the pixel and this information is directly linked to the photons received, not to the noise. However, the drawback is that the measurement becomes very difficult when the photon stream becomes high: the counting electronics can no longer distinguish the photons from each other if they are almost simultaneous: two simultaneous photons give rise to a single counting pulse.
To obtain advantages similar to those of photon counting without the drawbacks, WO 2007003577 has already proposed the use of a circuit operating by integration of electrical charges generated by the photodetector of the pixel but directly converting inside the pixel the quantity of charges into a digital value. This circuit consists of an integration stage, with an integration capacitor, which receives the charge current obtained from the photodetection, a threshold comparator and a counter; the threshold comparator detects that an elementary quantity of charges −Q0 has been received from the photodetector, it switches over and increments the counter by one unit (the minus sign is used arbitrarily, it corresponds to the fact that in practice the integration capacitor, precharged, sees its charge decrease all the more as the pixel receives a greater stream of X photons); concurrently with this switchover, the comparator triggers the injection of an opposite charge +Q0 into the capacitor; this returns to its initial charge while continuing to receive the charges from the photodetector; the counter therefore counts the bursts of charges −Q0 successively received by the pixel during the exposure period; this digital content then represents the quantity of photonic energy received during this period; it is this digital quantity which is supplied as output from the pixel.
FIG. 2 represents a schematic diagram of such a reading circuit. The integration capacitor is denoted Cint, the threshold comparator COMP1, the counter CPT1, and the +Q0 charge reinjection circuit CFB. The charge current generated by the photodetector is denoted Idet, and the voltage generated at the terminals of the integration capacitor Cint is Vint. This voltage is in sawtooth form if the current generated by the photodetector is constant since the voltage of the terminals of the capacitor periodically returns to its initial value.
When the radiological detector is not illuminated by X- or gamma-rays, the photodetector does, however, supply charges which correspond to its non-zero dark current. The dark current is a leakage current which is inevitable in the standard photodetectors, whether photodiodes or phototransistors for example.
The counter is therefore incremented regularly, even if slowly, in the absence of X or gamma illumination. The capacitor Cint is discharged by Q0 then recharged by Q0 periodically.
When the X illumination is triggered, the counting rate speeds up (except, obviously, for pixels which would be completely masked by a substance opaque to the incident rays). The counting speeds up all the more as the pixel is more strongly illuminated. It even speeds up considerably in undesirable situations in which a photon accidentally passes through the conversion structure 12 without being absorbed, and is directly absorbed into the CMOS reading circuit; such a photon can hit the reading circuit in a sensitive spot like the charge integration node; it then generates a large burst of electrical charges which will be dispelled only by a multiplicity of Q0 charge reinjections returned by the feedback circuit CFB. Other circumstances can even generate changes of counting rate as will be seen below.
To improve the possibilities offered by the detectors (and in particular the detector matrices) in which each pixel comprises a reading circuit with, on the one hand, a comparator switching over each time a charge increment arrives resulting from the integration of a charge current generated by the lighting and on the other hand a counting circuit for counting the number of switchovers of the comparator, the present invention proposes to incorporate in the reading circuit of each pixel a circuit for analyzing the rate of the switchovers of the comparator, this analysis circuit acting on the counting circuit to modify its operation according to the result of the rate analysis. The rate is analyzed on the basis of the observation of the time intervals that exist between successive switchovers.
In a first variant, if the rate is less than a bottom lower threshold value, provision is made for the content of the counter not to be incremented. On the other hand, above this threshold, the content of the counter is incremented. This drop below the bottom threshold is, in effect, the sign that no X or gamma illumination is probably present and that only the leakage currents are acting. This variant is useful, for example, for synchronizing the counting with the start of the illumination.
In a second variant, provision is made for the reading circuit to comprise two counters which are used alternately depending on whether the measured rate is above or below a threshold; this makes it possible, for example, not to lose the information added to the dark current, while clearly separating it from the information resulting from the X or gamma illumination.
In a third embodiment, compatible with the previous two, the counting is interrupted if the counting rate accelerates abruptly, indicating, for example, the impact of an undesirable photon directly in the CMOS reading circuit; the counting is restored when the effect of this impact, which is in theory occasional and rare but which could disturb an image, disappears.